Electronic frequency modulator



Sept. 9, 1952 G. R. CLARK 2,610,318

ELECTRONIC FREQUENCY MODULATOR Filed Dec. 5, 1947 2 SHEETS-SHEET 1ATTOJEIVEY Patented 9,

E EornoNro FREQUENCY MODULATOR.

Gilbert It. Clark, Nutley, N. J.,

. national Standard Electric Corporation,

assignor to Inter- New York, N. Y.,.'a corporationpf DelawareApplication Decem er a, 1947; Serial Not 7 9,415 j i j is Claims.(01.332-305 This invention relates to modulation systems and morespecially to improvedmethods' and means for modulatingthe frequency of asustained' alternating voltage, such as a carrier, in accordance withsignals; i

vA principal object of the invention is to provide an improved methodand apparatus for producing- ,relatively large frequencyswingswithjrespect to ;a mean or center 'fre'quency, and with arelatively- ,high order of frequency stabilityfboth -inthe .centerfrequency and the frequency swings corresponding to predeterminedsignals. v "Another principal object is-to provide an improved frequencyis useful in the modulation of any carrier whetheror radio frequencys'pecin the audio frequency trums. o

In general, prior knownmethods of direct electronic frequency modulationcan be divided into two categories. In one category; the carrier ismodulated by means of a signal-controlled reactance which forms part ofthe frequency-determining oscillatory circuit of thecarrier source. Anexample of one such arrangement is the reactance tube method, whereinthe signals control the reactance of an electron tube which, isconnected as part of the frequency-determining circuit. Another.such'example is the use of the Miller capacity effect to control acarrier frequency; j In'the second category of known frequencymodulation arrangements, there is employed a grid -controlled vacuumtube which is operated as a signal controllable resistance element inseries with a reactance, to control thereby the magni- :tlld and phaseangle of the combined series'impe'dance of the tube and'reactance;

One of the serious disadvantages of these prior known methods offrequency modulation is that the frequency stability is dependent, inlarge measure, upon certain inherent characteristics of the tubes, whichcharacteristics areofunctions of tetube plate current. Among thesefunctions may be mentioned transconductance, amplification factor, anddynamic plate resistance. However, as is wellknown, thesecharacteristics for any given'tubeare not rigidly related to the platecurrent of the tube, and very often one or more of the characteristicsvaries during the life of the tube, or under certain operatingconditions. In fact, variations may frequently be encountered betweensupposedly identical tubes of the sametype number, or physicalconstruction; The variability of the tube characteristics is reflectedas a corresponding instability in the frequency of the oscillator withwhich the tube modulation arrangementwhich cooperates, and it alsoaffects themodulation characteristics which relate input signalvoltageto outputfrequency' modulation.

Accordingly, it is another principal object of this invention to providea frequency modulation method and system whose operation is, withincertain :limits, not deleteriously affected by-vari-v ation in theabove-noted or other characteristicsof the electron tubes. f

Another object is tov provide a method andfapparatusfor effectingfrequency modulation b electronically switching in and out of afrequencydetermining network, and in accordance with signal ormodulating voltages, a reactance of constant value, or resistance ofconstant value, or a combination of reactanc'e and resistance of con-'stant value, the ratio of in and out switching times determining theaverage effect upon the frequency of the associated oscillator. Afeature of the invention relatesto a fre'' quency modulationsystememploying an oscillatorwith a tank circuit or the like, afrequencydetermining element of which is effectively switched in and outof circuit electronically and atan averaged rate determined by themodulating potentialg I I Z X Another feature relates to a frequencyvmodu lation system havin a frequency-determiningelement whichiscelectronically switched in and out eifectively'to modulate a carrier;in conjunc tion with circuit arrangements for rendering thefrequencystability of the modulated, carrier substantially independent of suchfactors as tank circuit A, C. voltage and the like. I I iAnother'feature relates to an improved fre quency modulator employing'anoscillator of the resistance-capacitance network control type, whereinthe frequency-determining elements areall of fixed value buttheir'eifect on the oscillator is controlled by electronic switchingmeans.

Other features and advantages not particularly set forth will appear'asthe ensuing'desqription I proceeds. o Inasmuch as the invention is'not'limited to any particular kind of intelligence orcontrol signal tobe transmitted, the invention willbe illustrated in a generic way so faras the signal source is con-j, cerned, and the component parts of thesystem whose individual operations are well-known in the art will beillustrated schematically, for the purpose of simplifying the drawing. V

. Accordingly in-the-drawing.

Fig l is a schematic wiring diagram of a frequency modulation systemembodying the inven- 5 tive concept.

Fig. 2 shows a series of graphs explanatory of the operation of Fig. 1.

Fig. 3 is a modification of Fig. 1.

Figs. 4, 5 and 6 are further and respective modifications of Fig. 1.

Referring to Fig. 1, the block l0 represents any well-known source ofsignal or control voltages which are to be converted into correspondingfrequency-modulated carrier. Thissource is generically represented as ahigh impedance source comprising, for example, a battery H and apotentiometer l2, whose adjustable element can be moved to determine thedesired signal. For example, the source 0 may be any well-known tele-..

metering transmitter or it may be a device such as a facsimiletransmitter, wherein the'resistan'ce l3 represents the load resistor ofthe light responsive phototube.

In fact, the resistance I2" may represent the load resistor of anywellknown signal source. The signals from source 1.0.- are applied, tothe nput t rm na l4, l5, ofa modulationpontrol unit I6, and thence to anoscillator unit l1, and the frequency modulated si nals aretaken off, atthe output terminals [8, [9. While the drawing shows an oscillator ll ofthe-double tube type, it will be understood that anyother Well-knownelectron tube oscillator maybe. employed- 7,

The input signals at terminals I4, I5, are applied. to the control gridof any well-known electron tubeamplifier 2!, and while the drawingshQwsa triode, any multi-grid electron tube can be employed. The tube 2| isshown connectedso as to act as a so-called cathode follower, wherein,the input signals appear as po-. tential variations across the cathodefollower load resistance 22 which is connected between the cathode 23and ground; and if desired, in series with another but higher resistance24 to prevent excessive dissipation within the tube since resistance .22preferably has a low value. Inethe conventional manner, a highresistance 25.,isconnected between grid 20 and ground. Insteadof using aseries current-limiting.resistor 24,:the cathode follower resistance 22alone-may be used and the positive D. C. operating potential applied toanode 26 from the D. C. plate supplysource 21 may be correspondinglyreduced.

either case, a proportionate part of, the potential obtained from thesource |0 appears'across resistor 22. I I

. The oscillator I! may be of the type having for, example, a firstgrid-controlled tube 28 whose cathode 29 is suitably biased bycathoderesistor 3Q, and whose output anode 3| is coupled by condenser 32to the control grid 33 of a feedback contro1tube-34. In the conventionalway, a grid leak resistor 35 is connected between grid 33 and ground.The anode 36 of tube 34 is coupled for oscillation feed back actionthrough condenser 31 and resistor 38 to the control grid 39. Thefrequencyof oscillator I1 is determined by the usual tuned oscillatoryor tank circuit comprising inductance 40, shunt condenser 4| and alsoincluding shunt condenser 42 in'a manner which will be described. Thevalues of V the various components 'of the tank circuit are chosen sothat when no voltage is applied to condenser 42 from the modulator unitIS, the oscillator generates a sustained and fixed frequency at thelower end of its modulation range, as represented for example by thegraph 43 (Fig. 2). In order to control the effective switching .in andout of the condenser '42 and thereby controlling the frequency of thecarrier at the output terminals 18,

l9, 2. pair of oppositely poled diodes 44, 45, are bridged across theresistor 22; and the anode of diode 44 as well as the cathode of diode45 are connected in parallel to the condenser 42.

Let it first be assumed, for purposes of explanation, that no voltage isdeveloped across the resistor 22, that is to say, assume that the tube2| is removed from its socket. Referring to Fig. 2, which shows. thetank circuit voltage as a function of time, the diode 45 is conductivebetween the points 0 and E, whereas the diode 44 isconductive betweenthe points E and F. Since this A: C. tankvoltage renders diodes 44 and45 conductive throughout every alternate half cycle, the fact is thatthe condenser 42 is effectively in p'arallel withthe tank circuit units[0 and H substantially all the time, and the oscillator frequency is atthe lower limit of the modulation range as .represented by graph 43.

Assume now that the tube 2| is replaced in its socket and thatpotentiometer I3 is adjusted so that the potential which appears acrossresistor 22 is more than twice the peak value of the A. C. tank circuitvoltage. During. the negative peak of the A. C. tank voltage, condenser42 acquires acharge through the conductivity of diode 45. Afterthisfirstnegativepeak, a steadystatecondition exists, during which nofurther current flows into condenser 42 because the potentialacross'resistor 22 maintains the cathode 46 too positive with respect toits anode and therefore the initial charge retained by condenser 42blocks diode 45 against further condition. Under this condition,condenser 42 1s effectively disassociated from the tank circuitunits 40,4|, and the oscillator=|1 generates .atthe upper limit of the modulationrange as represented by graph 47.

Graph 48 represents an intermediate condition, which exists when thepotential'across resistance 22 equals the peak value of the A. C. tankvoltage. Atpoint O, condenser42 has no charge ofits own and neitherdiode 44 nor diode :45 is conductive. When the :A. C. voltage reachespoint A, diode 45 starts to conduct and continues conductive through topoint Thiscauses condenser 42to acquire a charge, which at point I-I,equals the peak value of the A. C. voltage, at which time the plate ofcondenser 42 which .is connected to the diodes, is positive with respectto the plate of condenser 42 which is connected to grid 39. From pointsvI-Ito J -of,c urve 48,..neither diode 44 nor, diode'45 is conductive,and therefore curve'48 be-' tween points H and J resembles curve)between p oints'B and .M. At point J, however,.the. po-' tential of thecharge in condenser '42 balances the potential across'resistor '22. Asthe A. C. tank voltage continues positivefrom point J, diode 44 isconductive. until.point.'K,bywhich time it has neutralized the chargeincondenser '42 thereby restoringthe initial condition.

Inasmuch as thefrequency of. the-effectiveelectronic.- switching bydiodes .4'4'and 45 may have any value higher than. the highest wantedfre' quencies in the signal voltages from source [0, the embodiment ofFig.1 aswell as the remaining embodiments to, be described, are arrangedto, have twosw-itcliing cycles forj each cycle of the frequencyof'oscillator H. v

A system as illustrated .Fig. .1, has been found to possess asatisfactory degree of frequency sta-' bility and amplitudestability.While the amplitude'of the A. .C. tank circuit voltage may not be,exactly constant for allsettings of potentiometer I3or for all valuesofinput signals, nevertheless. it varies in a consistentv mannerunaffected by pedance at resonance to that impedance plus the feed-backresistance 38, and. all the parameters involved can therefore bedesigned tohave consistentvalues. V f

Referring to Fig.3, there is; shown a modification of the;system of Fig.2', wherein the signal modulations from the source It. instead of beingapplied to produce variations of D. C.;, voltage across resistor 22,,areapplied in, any well-known manner to vary the feed-back-between plate 36and grid 39 by means for example ofa-control tube 50. Thus, when thecontroltube -56 draws current through-load resistorjs, the, resultingIR' drop lowers the supply voltage applied to plate; Therefore, thelimited alternating voltage- 36. across resistori le is reduced inamplitude as is the alternating voltage across tank circuit 46, 4 I. Inthis embodiment, the voltage across the resistor 22 is at afixed value,for example by means or battery* 5|,"' and frequency modulation isachieved by varying the amplitude of the alterhating Voltage. Thereforetube 50 is used in cooperation with unit [6 for modulation control.Theremaining elements of the system of Fig. 3 are identical with thosebfFig.1 and their functioning is the samefso that further descriptionthereof is not required at this "point.

Referring' to'Fig. a, there is shown a modification ofFig. 1, whereinspecial means are provided for maintaining amplitude stability. In Fig.4, the parts which function the same, as those of Fig. 1 are designatedby thesame numerals. However, for simplicity, the oscillator His shownas of the single tube type; The rectifiers 44 and 45 maybediode'rectifiers or they may be any other form of rectifier such asselenium, copper oxide, germanium or other contact rectifier.

Likewise, the additional rectifier 52 may be of the diode type or anyone of the well-known contact rectifier types. When the peak value ofthe alternating current tank voltage exceeds the D. C. potential atpoint 53, rectifier 52 becomes conductive on positive peaks andincreases the losses of the tank circuit. This inhibits the oscillatoramplitude from building up beyond'this point. The function of condenser54 is to provide a radio frequency by-pass for the circuit whichincludes the input terminals l4, l5, and the resistor 22. It will benoted that in Fig. 4, the source I0 is schematically represented as astandard microphone input for producing voice frequency signals. Thecenter frequency of the oscillator I! is substantially independent ofplate voltage variations. The D. 0. Voltage across resistor 22determines the center frequency for a given signal input amplitude, butit also determines the amplitude of the voltage from oscillator ll.These two eifects cancel each other as regards any change in the centerfrequency.

While the systems of Figs. 1, and 4 produce satisfactory linearitybetween input signals and output frequency modulations, it may bedesirable to employ a special linearity control for this purpose. Suchan arrangement is schematically illustrated in Fig. 5. In this figure,the block 55 represents the units I6 and ll of any of the systoms ofFigs. 1, 3and 4. However the device 55 is connected to the signal sourceIn through any well-known amplitude modulation amplifier 56.

Alportion ofv the output of the .devicei 5-5, is then? applied. tov any.well-known frequency modulation demodulator 51, .whichiapplies anegative feed-' back potentialwhich is, combined with the originalsignal from sourcell 0 and applied to the control electrode of theamplifier 56 in the manner well-known in negative feedback systems.

. Inthe-embodiment of Figs. 1, Sand 4, the free quency modulation hasbeen controlled by theeifective electronic switching of the condenser42, It will be understood, however, that frequency .modulation canlikewise be attained by,

similar: switching in and out of inductances, resistors, or impedancenetworks. Fig. 6 illustrates the frequencymodulation principle of thisinvention as applied toa resistance-capacity oscillator,

whereinthe electronic switching is eifected on a:

resistance contained in the frequency-determin ing -;networks. i In thisembodiment, there "is shown asymmetrical arrangement of two oscillatortubes 58, 59, although the invention can be equally.- well applied toasingle oscillator tube; The output mesh of tube 58 comprising condenser60 and resistors 6|, 62 and is connectedby means of conductor '63 to asecond resistance-capacity mesh comprised of resistor 64 and condenser65. Because of thesymmetrical arrangement of the two tubes, a singlecondenser65 can be employed, although if desired, the condenser 65 maybe replaced by two series condensers, each having twice the capacitanceof condenser '65 andwith their: common or center connection grounded;-

This second mesh'introduces an equal and-opposite phase shift tothatcaused by the first mesh, 66, 6|, 62, so that the signalat the gridoftube 59 is in phase with the signal at the plate of tube 58. Agridresistance 66 may be connected between thecontrol grid of tube 59and the second mesh so as. to suppress parasitic oscillation.

Tube 53causes an degree reversal of the sig;

the transformer .66 to the diodes or rectifiers 61, v

68. If no modulating signal is present at the point 69, diode 61conducts throughout the positive half cycle of the oscillator voltageacross resistor 6|, thereby shunting this resistor half of the time. Thesignal across the corresponding resistor 10 for the tube 59 is oppositein phase to that across resistor 6|, and diode 68 shunts'resistor 10 forsimilar half cycle periods. Depending upon the modulating signalpolarity supplied at points 63, diodes 61 and 68 are conductive for moreor less than half of the time, thereby correspondingly increasing ordecreasing the oscillator frequency from its mean or center value.

While certain particular embodiments have been disclosed and describedherein, various changes and modifications may be made therein withoutdeparting from the spirit and scope of the invention. 1

What is claimed is:

1. A frequency modulation arrangement for,

converting variable amplitude signal into correspondingfrequency-modulated carrier, comprising a variable carrier frequencygenerator, a tuned oscillatory tank circuit for said generator andincluding a frequency-determining reactive circuit element which isarranged to be switched lntaand out or effective relation said tank.cirnuitq a pair: of; rectifiersconnected'to said elegmentito effectsaidswitching, and means to con.- trol theiconductiviti'esof saidrectifiers' jointly' by said'sig nals and by the'alternating peakvoltageof's'aidz'tankcircuit.

2. A frequency modulation arrangemem:ac.- cordingto claim 1 in whichmeans are provided to ma'i'ntainthe alternating peak voltage of saidtanl'c circuit at asl'zbstantially uniform level;

3. A frequency modulation arrangement for converting variableamplitude-signals into correspending frequency-modulated carrier,comprismg a variable frequency generator, a resistance-- capacitancenetwork for controlling the ire-- quency' of said generator, a pair ofrectifiers for switching a frequency determining capacitance element ofsaid network into and out of effective frequency-determiningrelationwithsaid generator, and means to control the conductivities of saidrectifiers by the said variable amplitude sign'als andby'the' wavesfrom'said generator to eflectsaid switching.

4; In a frequency modulating device, an oscillator including afrequency-determining tank circuit having a'grounded and an ungroundedend, a capacitor directly connected to them;- grounded end of said tankcircuit, electronic switch means for intermittently connnecting-saidcapacitor to the grounded end of said'tank circult in parallel therewithto vary the resonant irequencv thereof said sw'itch means including anelectron discharge device and 'a cathode -resistor'series connected toground for-derivation of a'n'routput signal thereacross in response tovariableampli'tude signals fed to said discharge device a pair ofrectifiers, means connecting-one of said re'ctifiers between saidcapacitor'and a point intermediate said cathode and said cathoderesistor, and means connecting the other of said rccttfiefs 1n. 'opnsite ..polarityibetween; said. Q91?" pacl torzandgroundr V 5. A.freq'uenc-ymodulation circuit.- for; convene ing variable, amplitudesignals into. a corresponding frequency modulated carrier. comprising" avariablecarrier frequency generator. 2. tuned oscil l'atorytankzc-ircuit. for said generator and inciudinaa frequencydeterminingreactive. element. mcans connecting said reactive element forswitching into and out of effective relation with said-tank circuit,said means including a pair of recti tlens connected to said element toeffect said witching in response to both said variable amplitude signalsand. to: energy: from said generator, and means maintaining thealternating peakvoltage ofsaid tankcircuit at: a substantially-unisform-level.

6; The device oi -claim, 5 includingmeans for switching said elements inandoutv of the circuitat an averaged rate determined bythe modulat ing"potentials.

GILBERT'R.

REFERENCES CITED The. following references are of record in the. file.of this -patent:

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